The cost to build a utility-scale photovoltaic solar farm has dropped 80 percent since 2009, while wind farm costs have dropped 60 percent, according to the Brookings Institution.

The economic competitiveness of natural gas and renewable energy is more meaningful at this particular moment, because flat electricity demand is driving utilities to seek the cheapest possible options for power generation, Brookings analysts say.

As domestic coal consumption decreases, exports are also weak, falling for the third consecutive year in 2015.

This is largely due to the exploitation of the cheapest and easiest-to-mine coal sources, requiring coal companies to turn to more difficult and costly mining operations.

Overall productivity has rebounded somewhat in the past three years, but this may only be the result of the closing of less-efficient mines, and a one-time boost from the implement of certain new technologies.

A decrease in coal use for electricity generation is good news for drivers of electric cars, because these cars get cleaner alongside the sources of electricity used to charge them.

But it also leaves open the question of how to stimulate the economies and preserve jobs in coal-producing states, the answer to which may well be transitioning to different industries altogether.